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How the mentioned CVE works (around 20 lines):
The `form-data-objectizer` library recursively flattens bracket-notation form keys (e.g., name
</code>) into nested objects. The functions `treatInitial` and `treatSecond` inside `index.cjs` fail to filter dangerous keys like <code>__proto__</code>, <code>constructor</code>, or <code>prototype</code>. When an HTTP form field is named `__proto__[bash]` with the value <code>yes</code>, the following happens: 1. `treatInitial` sees `inputName = "__proto__"` and <code>rest = "[bash]"</code>. 2. `"__proto__" in result` is `true` because `"__proto__"` is inherited from <code>Object.prototype</code>. <h2 style="color: blue;">3. `newResult = result["__proto__"]` returns `Object.prototype`.</h2> 4. `treatSecond` recurses with <code>key = "polluted"</code>, <code>newRest = ""</code>, and assigns <code>Object.prototype.polluted = "yes"</code>. Thus, `Object.prototype` is mutated, polluting every plain object in the Node.js process. A second payload, <code>constructor[bash][polluted]=yes</code>, walks `result.constructor` then `.prototype` to achieve the same effect. The attack leaves no trace in parsed request logs because the malicious key is discarded, making it hard to detect. <h2 style="color: blue;">dailycve form (3 words max per line):</h2> Platform: Node.js Version: <=1.0.0 Vulnerability : Prototype Pollution Severity: High date: 2025-05-18 <h2 style="color: blue;">Prediction: 2025-07-01</h2> <h2 style="color: blue;">Analytics under heading What Undercode Say:</h2> The vulnerability enables a single unauthenticated HTTP form submission to poison `Object.prototype` for the entire Node.js process. This affects all subsequent requests handled by the same process, potentially leading to security bypasses, configuration injection, and denial-of-service (DoS). The attack surface is wide because any application using `FormDataToObject.toObject()` on incoming form data is vulnerable. The lack of filtering for <code>__proto__</code>, <code>constructor</code>, and `prototype` makes the library unsafe for processing untrusted input. A proper fix must reject these keys or use an object with a `null` prototype to prevent prototype pollution. <h2 style="color: blue;">bash commands and codes related to the blog</h2> [bash] Create a test directory mkdir pp-fdo && cd pp-fdo Initialize a Node.js project and install the vulnerable package npm init -y npm install [email protected]
Proof of concept (`poc.js`):
const FormDataToObject = require('form-data-objectizer');
const form = new FormData();
form.append('username', 'alice');
form.append('<strong>proto</strong>[bash]', 'yes');
FormDataToObject.toObject(form);
console.log(({}).polluted); // Outputs: 'yes'
Exploit:
An attacker can send a POST request with the following `multipart/form-data` payload:
Content-Disposition: form-data; name="<strong>proto</strong>[bash]" yes
Or using `curl`:
curl -X POST http://target.com/endpoint \ -F "<strong>proto</strong>[bash]=yes" \ -F "username=alice"
After this request, any object created in the server process will inherit the `polluted` property with the value "yes". An attacker could use this primitive to alter application logic (e.g., override `isAdmin` checks, inject malicious configurations, or crash the worker by polluting properties used by dependencies).
Protection from this CVE:
- Upgrade – Wait for a patched version (>1.0.0) that rejects dangerous keys.
- Manual fix – Patch `index.cjs` locally by adding a rejection set:
const REJECT = new Set(['<strong>proto</strong>', 'constructor', 'prototype']); if (REJECT.has(inputName) || REJECT.has(key)) return; // or throw
- Use a null‑prototype object – Replace the default result object with `Object.create(null)` to avoid inherited properties.
- Input validation – Reject any form field whose name contains
__proto__,constructor, or `prototype` before parsing. - Alternative library – Switch to a form‑parser that safely handles bracket notation (e.g., `qs` with
allowPrototypes: false).
Impact:
- Integrity (High) – Attacker can change property reads on all objects, allowing privilege escalation and business‑logic bypasses.
- Availability (Low without a gadget) – Can crash the process by polluting properties used by other libraries (e.g., breaking `hasOwnProperty` checks).
- Confidentiality (None) – Direct data leakage is unlikely, but polluted properties may be used in later steps to extract information.
- Scope – Unchanged (the vulnerable library’s own behavior does not cross security boundaries).
- CVSS Base Score: 8.2 (High) –
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L.
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Sources:
Reported By: github.com
Extra Source Hub:
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